The Mississippi River which is over 2300 miles long is the second-longest river in the United States.

A、The Mississippi River which is over 2300 miles long is the second-longest river in the United States. (No commas)

B、The Mississippi River, which is over 2300 miles long is the second-longest river in the United States.

C、The Mississippi River which is over 2300 miles long, is the second-longest river in the United States.

D、The Mississippi River, which is over 2300 miles long, is the second-longest river in the United States.

正确答案：The Mississippi River, which is over 2300 miles long, is the second-longest river in the United States.